1. Field of the Invention
The present invention relates to a calibration method of an image scanning system, and more particularly to a calibration method utilizing differences between calibration data.
2. Description of the Prior Art
An image system, like a scanner, typically possesses problems such as image defects or distortion of intensity owing to (a) the photo-response non-uniformity of the light source, the mirror, the lens and the charged-coupled device (CCD), (b) the aging of the system and, (c) the effects resulting from a dusty environment. Therefore, image information of an object captured by the image system is required to compensate or calibrate.
Referring to FIG. 1, which is a schematic cross-sectional view of a prior flat bed scanner 101. A white calibration plate 102 is provided outside a document sheet read area. A document sheet 104 is mounted on a document sheet table glass 103 and secured by a cover 112. The white calibration plate 102 is read prior to the reading of the document sheet 104 and the read signal is stored in a memory. When the document sheet 104 is scanned, a linear light source 105 illuminates on the document sheet 104. The reflected light from the document sheet 104 is directed to mirrors 106, 107 and 108 and directed to a charge-coupled device (CCD) 110 through a lens 109. An output of the CCD 110 as a document sheet read electrical signal is sent to an A/D converter (not shown) established within a signal processing unit (not shown) to convert the electrical signal into a digital signal. For a color image-scanning device, the charge-coupled device 110 includes three linear sensors, respectively R linear sensor, G linear sensor and B linear sensor. Each of the three linear sensors includes a plurality of photo-sensing elements, and each of the photo-sensing elements provides a sensing value for one pixel.
FIG. 2 is a flow chart of a general scanning process. Initially, at step 201, scanning a calibration plate to capture calibration data and the calibration data read from the calibration plate is stored in a calibration memory. Continuously, at step 202, a central processing unit (CPU) of a host computer reads the calibration data of each scan line of the calibration plate from the calibration memory, and proceeding computation to generate calibration data corresponding to each photo-sensing element of the linear sensor. At step 203, the calibration data are restored in the calibration memory. At step 204, a normal scan action is performed. A document sheet is scanned for capturing the image information thereof and simultaneously reading the calibration data corresponding to each photo-sensing element to calibrate the image information.
In the above calibration method, the calibration data read from the calibration plate is directly stored in the calibration memory. As an example, for a color image reading device including R, G, B three linear sensors, each of R, G, B linear sensors comprises 10000 photo-sensing elements, and the output of each photo-sensing element is converted to 8-bit digital signal and then stored in the calibration memory. In this case, a volume of the calibration memory for storing calibration data is 8*3*10000=240000 bits. This prior calibration method spends a lot of storage volume of the calibration memory and much time to read calibration data. Both of which adversely affect effectiveness of image capture and manufacturing cost for a high-speed scanning system and a high-pixel scanning device.
Accordingly, it is an intention to provide an improved calibration method to overcome the above drawbacks.